Silbury Hill is a prehistoric artificial chalk mound near Avebury in the English county of Wiltshire. It is part of the Stonehenge, Avebury and Associated Sites UNESCO World Heritage Site, and lies at grid reference SU099685. At 40 metres (131 ft) high, Silbury Hill – which is part of the complex of Neolithic monuments around Avebury, which includes the Avebury Ring and West Kennet Long Barrow – is the tallest prehistoric human-made mound in Europe and one of the largest in the world; it is similar in size to some of the smaller Egyptian pyramids of the Giza Necropolis. Its original purpose however, is still highly debated. Several other important Neolithic monuments in Wiltshire in the care of English Heritage, including the large henges at Marden and Stonehenge, may be culturally or functionally related to Avebury and Silbury.
Composed mainly of chalk and clay excavated from the surrounding area, the mound stands 40 metres (131 ft) high and covers about 5 acres (2 ha). It is a display of immense technical skill and prolonged control over labour and resources. Archaeologists calculate that Silbury Hill was built about 4,750 years ago and that it took 18 million man-hours, or 500 men working for 15 years (Atkinson 1974:128) to deposit and shape 248,000 cubic metres (324,000 cu yd) of earth and fill on top of a natural hill. Euan W. Mackie asserts that no simple late Neolithic tribal structure as usually imagined could have sustained this and similar projects, and envisages an authoritarian theocratic power elite with broad-ranging control across southern Britain.
The base of the hill is circular and 167 metres (548 ft) in diameter. The summit is flat-topped and 30 metres (98 ft) in diameter. A smaller mound was constructed first, and in a later phase much enlarged. The initial structures at the base of the hill were perfectly circular: surveying reveals that the centre of the flat top and the centre of the cone that describes the hill lie within a metre of one another. There are indications that the top originally had a rounded profile, but this was flattened in the medieval period to provide a base for a building, perhaps with a defensive purpose.
Silbury Hill is located in the Kennett Valley. It is close to the A4, between the towns of Marlborough and Calne, also the route of a Roman road which runs between Beckhampton and West Kennett and runs to the south of the hill. In 1867 the Wiltshire Archaeological and Natural History Society excavated the east side of the hill to see if traces of the Roman Road were underneath it. No traces were found and later excavations south of the hill located the road in fields to the south making a pronounced swerve to avoid the base of the hill. This was conclusive proof that the hill was there before the road - but the hill provided an alignment sight-line for the road.
The hill's vegetation is species-rich chalk grassland, dominated by Upright Brome and False Oat-grass, but with many species characteristic of this habitat, including a strong population of the rare Knapweed Broomrape. This vegetation has led to a 2.3 hectares (5.7 acres) area of the site being notified as a Site of Special Scientific Interest, this notification initially being given in 1965. The site is unique in that its slopes have 360-degree aspects, allowing comparison between growth of the flora on the differently-facing slopes of the hill.